Two of the available methods to program a programmable read-only memory (PROM) include use of metallic fusible links or its opposite, the antifuse. An unprogrammed antifuse is non-conducting, and it is programmed by changing it to a conductive or low resistance state so as to close a circuit between the bit line and the array device. A fuse link, on the other hand, is conductive when unprogrammed, and it is programmed by changing it to a non-conducting state so as to open or break the circuit between the bit line and the array device. In each case, the programming is effected by imposing a high voltage on the device causing a current to flow through the link creating heat in the link. In the fuse link, heat opens the circuit; wherein, in the antifuse, the heat creates a conductor.
In the case where the memory device is a metal oxide silicon field effect transistor (MOSFET), it has typically been known to insert the antifuse in series with the source node. This has an undesirable consequence often referred to as "body effect", which occurs when there is a reverse bias on the source-substrate, V.sub.SB. This bias can occur during programming by the high voltage due to the possibility of high resistance in the antifuse, i.e., 1,000-4,000 ohms.
The resultant voltage drop during programming through this resistance reduces the gate-to-source voltage V.sub.gs causing a reduced drain current, I.sub.D. This reduced drain current may not be adequate to properly heat the antifuse, thereby forming a contact having high resistance.
It is, therefore, desirable to fabricate a multiplicity of small outline antifuses in a PROM MOSFET drain to avoid the reverse bias of the source and provide for a high-drain current during the programming operation.
In this PROM, the MOSFET transistor having a shorted antifuse is operational, i.e., it can be conducting, simulating a one or can be an open circuit because of the non-conducting antifuse in the drain, simulating a zero.